The most widely known process in current use for the production of ethylenic compounds from aldehydes or ketones comprises reacting these carbonyl compounds with a phosphonium halide, for example triphenyl alkyl phosphonium bromide which itself is obtained by reacting triphenylphosphine with an alkyl bromide. Although this reaction takes place satisfactorily, the starting triphenylphosphine is converted into triphenylphosphine oxide which has to be reduced for recycling. Another method of synthesising ethylenic compounds from aldehydes or ketones comprises reacting the aldehydes or ketones with sulphone, for example with phenyl benzyl sulphone, to form an alcohol sulphone (Journal of Chemical Society PERKIN I, page 1166, 1973) which is then converted into an ethylenic sulphone by treatment with a dehydrating agent such as phosphoric acid. The treatment of this sulphone with a reducing agent, such as aluminium amalgam or a metal hydride, results in splitting of the sulphonyl group and liberates the required ethylenic compound (Journal Chemical Communications, page 351, 1973). The advantage of this method, in which hydroxysulphones are formed as intermediate stage, is that it gives high yields, but unfortunately the successive stages of dehydration and desulphonation each necessitate a different agent and, on completion of dehydration, the ethylenic sulphone obtained has to be dehydrated in order to be able to subject it to the reducing splitting treatment.